The number of students who spent their last weeks of high school preparing for a final exam on the wrong Caesar has grown to 140 across nine schools, with the state’s education minister suggesting they would not be expected to sit the test.

But the minister, John-Paul Langbroek, could not clarify how those year 12 students would be tested, hours before their fellow budding classicists from around the state prepared to answer questions about their knowledge of Julius Caesar.

Students from nine schools stretching from Cairns, in the state’s far north, down to Brisbane had been preparing for an external exam on his adopted son and heir, Augustus.

They learned of the curriculum misadventure this week – hours before the exam was to be sat on Wednesday afternoon.

Langbroek said he was “very unhappy” to learn of the situation, indicating heads would roll at the Queensland Curriculum and Assessment Authority (QCAA), adding that news of the mistake would be “extremely traumatic” to students and their parents.

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“I want to reassure those students – and their parents, and their teachers – affected that we will be making every investigation into how this happened,” he said on Wednesday.

The minister tried to reassure students that the bungle would not be “the end of the world”, saying that the 75% of the course mark for which they had already been tested would be “scaled up” to make sure they were not disadvantaged.

Asked whether that made Wednesday’s test “null and void”, or if not, what the exam would look like, Langbroek said he did not have the details.

“I’m not sure about the actual process of what is happening with the examination,” he said.

“But I can’t see how anyone would be expected to do an exam having had two days’ preparation.”

Langbroek said the department was yet to find the cause of the error.

“I have directed the director general of the Department of Education to urgently investigate how the QCAA communicates with schools to implement syllabus changes,” Langbroek said.

The minister said he would seek to “make sure” the resources given to and planning done at the QCAA was “appropriately guided in future” to avoid a repeat of the bungle. He noted the board had already seen “some changes” since the Crisafulli government came to power a year ago.

The schools affected are: Brisbane state high school, Flagstone community college, Meridan state college, Redcliffe state high school, Yeronga state high school, Saint Teresa’s Catholic college, West Moreton Anglican college, James Nash state high school and Kuranda district state college.